1. Field of the Invention
This invention relates to an objective lens used in an endoscope.
2. Description of Related Art
Endoscopes have been used for observations of parts in which it is difficult to make external observations, like treatments and/or diagnoses of interiors of bodies of patients in a conventional field of medicine. Recently, a sleek design of the endoscope, represented by a nasal endoscope, has been needed and even for the objective lens used in the endoscope, downsizing of its outside diameter has been desired.
Since a conventional objective lens used in the endoscope has a very large angle of view, a retrofocus-type lens system in which a plano-concave lens element is generally used at the most object-side position of the objective lens has often been adopted. In the retrofocus-type lens system of this sort, the technique of designing a small objective lens is to impart a strong negative power to the plano-concave lens located at the most object-side position. However, when the first surface is configured to be flat, there is the problem that variation of the angle of view is increased. Additionally, in the retrofocus-type lens system, when the power of a front negative lens unit is increased to maintain the specification, the power of a rear positive lens unit must be necessarily increased.
However, conventional objective lenses for endoscopes in which the first surface is configured as a curved surface are disclosed, for example, in Japanese Patent Kokai Nos. Hei 08-334688 and Hei 02-188709.
The objective lens for endoscopes set forth in Kokai No. Hei 08-334688 includes, in order from the object side, a first lens element with negative refracting power of a meniscus shape; a second lens element of a plano-convex shape; a stop; a third lens element of a plano-convex shape; and a fourth lens element of a biconvex shape, and is designed to satisfy preset conditions described below and to configure the first surface as a convex surface.Bf′>1.5f′  (51)1.80<D/r2<2.40  (52)ν2<45.0  (53)ν3>50.0  (54)ν4>50.0  (55)where Bf′ is a back focus, f′ is the focal length of the entire optical system, r2 is the radius of curvature of the image-side surface of the first lens element, and D is the value of d2+(d3/n2)+d4, where d2 is air spacing between the first lens element and the second lens element, d3 is the center thickness of the second lens element, d4 is air spacing between the second lens element and the stop, and n2 is the refractive index of the second lens element.
The objective lens for endoscopes set forth in Kokai No. 02-188709 includes, in order from the object side, a first lens element with negative refracting power of a meniscus shape, with a concave surface facing the image side; a second lens element with positive refracting power of a meniscus shape, with a surface of the major radius of curvature facing the image side; an aperture stop; a third lens element of a biconvex shape with a surface of the major radius of curvature facing the object side; and a fourth cemented lens component with positive refracting power as a whole, and is designed to satisfy preset conditions described below and to configure the first surface as a convex surface.0.6<|f1/f|<0.70  (61)n2>1.60 ν2<45.0  (62)n3>1.60 ν3>50.0  (63)ν4<40.0  (64)n5>1.60 ν5>50.0  (65)where f1 is the focal length of the first lens element; f is the combined focal length of the entire system; n2, n3, and n5 are refractive indices, relative to the d line, of glass materials used for convex lenses of the second lens element, the third lens element, and the fourth cemented lens component, respectively; and ν2, ν3, ν4, and ν5 are Abbe's numbers of glass materials used for the second lens element, the third lens element, the concave lens element of the fourth cemented lens component, and the convex lens element of the fourth cemented lens component, respectively.